All living things are made of cells, which are the smallest units capable of performing life functions. There are two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells lack internal structures surrounded by membranes and are found in bacteria, while eukaryotic cells contain membrane-bound organelles and are found in plants, animals and other organisms. Both animal and plant cells contain a nucleus that holds DNA, organelles like mitochondria and chloroplasts, and a cytoplasm surrounded by a cell membrane. Plant cells also have a cell wall providing structure.